Step 1: Sterilize your canning jars and lids by washing thoroughly, then boiling for 10 minutes or running through a hot dishwasher cycle. Keep them hot until ready to fill.
Step 2: Prepare the fruits. Hull and quarter the strawberries. For the oranges, wash well, then zest one orange (avoiding the white pith). For the remaining oranges, carefully peel, remove as much white pith and membrane as possible, and thinly slice the segments. Collect any juice.
Step 3: In a large, heavy-bottomed pot or Dutch oven, combine the prepared strawberries, orange segments, orange zest, granulated sugar, and lemon juice. Add the optional vanilla bean if using. Stir gently to combine.
Step 4: Let the fruit mixture sit at room temperature for at least 30 minutes, or preferably several hours (or overnight) in the refrigerator. This helps draw out juices and begin to dissolve the sugar.
Step 5: Place the pot over medium-high heat. Bring the mixture to a rolling boil, stirring frequently to prevent sticking. Continue to boil, stirring occasionally, until the marmalade reaches its setting point (around 220°F or 104°C on a candy thermometer).
Step 6: Test for setting point using the cold plate test: spoon a small amount onto a chilled plate, let it cool for a minute, then push it with your finger. If it wrinkles, it's ready. If not, continue cooking and test again in a few minutes.
Step 7: Once the marmalade has reached its set, carefully remove any vanilla bean (if used). Ladle the hot marmalade into your hot, sterilized jars, leaving 1/4 inch of headspace.
Step 8: Wipe the rims clean, place the lids and bands on, and tighten to fingertip-tight.
Step 9: For long-term storage, process the filled jars in a boiling water canner for 10 minutes. Remove jars and let them cool completely on a wire rack for 12-24 hours. Check seals; lids should be concave. Store any unsealed jars in the refrigerator and consume within a few weeks.